drm/panthor: avoid garbage value in panthor_ioctl_dev_query()

'priorities_info' is uninitialized, and the uninitialized value is copied
to user object when calling PANTHOR_UOBJ_SET(). Using memset to initialize
'priorities_info' to avoid this garbage value problem.

Fixes: f70000ef ("drm/panthor: Add DEV_QUERY_GROUP_PRIORITIES_INFO dev query")
Signed-off-by: Su Hui <suhui@nfschina.com>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250119025828.1168419-1-suhui@nfschina.com